Sulfonated polyether ether ketone membranes cured with different methods for direct methanol fuel cells

Abstract

Direct methanol fuel cells (DMFC) have attracted considerable attention, since they offer numerous benefits, such as high efficiency, high power density, low or zero emissions and reliability. The proton exchange membrane (PEM) is one of the most critical components in the direct methanol fuel cell. Generally, PEMs with high proton conductivity and low methanol permeability are desirable for efficient fuel cell operation. Currently Nafion©R membrane, a perfluorosulfonate ionomer, is the major membrane used in polymer electrolyte membrane fuel cells (PEMFC). But Nafion©R membrane is a poor barrier to methanol crossover. In addition, the high cost of perfluorosulfonate membranes prohibits wide commercial application for PEMFC. Therefore it is important to find novel PEMs with low methanol permeability and low cost. Several new types of membranes have been prepared for use in DMFC. Among them are sulfonated polyether ether ketone (SPEEK) [1], sulfonated poly(arylene ether sulfone) (BPSH) copolymer [2], sulfonated or phosphonated polyphosphazenes [3, 4], poly(vinyl alcohol) (PVA)-crosslinked-polystyrene sulfonic acid (PSSA) [5], blend membranes [6, 7] and inorganicorganic composite membrane [8]. Recently, a few researchers found that the proton conductivity of PEMs varies with curing method [9–11]. Bauer et al. [9] reported that the conductivity of SPEEK membranes cured in boiling water for 4 h increased by a factor of ca 7 compared to the blank. McGrath et al. [10] also noted that the post-treatment of BPSH copolymers in boiling water produced higher proton conductivity and water absorption. These initiated our interest to understand the phenomenon and to see if curing affects methanol permeability of PEMs as well. In this paper, we report the influence of curing of the SPEEK membranes on their proton conductivity and methanol permeability. Sulfonated polyether ether ketone (SPEEK) with degree of sulfonation 38% (100% sulfonation corresponds to 1 sulfonic acid group per repeat unit) was prepared as described in [12]. Membranes were prepared by casting a 10 wt% SPEEK in dimethylformamide (DMF) solution on glass plates and then drying them at 60 ◦C for 6 h and 100 ◦C for 4 h. Afterwards, the membranes were cured by two different methods: (1) immersing in 1.0 M sulfuric acid solution at room temperature for 24 h and subsequently rinsing with